Question 1

A crucial step in the regulation of most bacterial genes occurs

Accepted Answer

The regulation of most bacterial genes primarily occurs at the stage of transcription initiation, where the decision to transcribe a gene into mRNA is made. Bacteria do not have nuclei, so processes like RNA splicing and nuclear export of mRNA (options B and C) are not applicable. Nonsense suppressors (option D) are involved in overcoming stop codons during translation, not in the primary regulation of gene expression.

Question 2

What term describes regulation of the trp operon that can occur even in trpR− mutants, thus suggesting that the mechanism is repressor-independent?

Accepted Answer

Attenuation is a mechanism of regulation of the trp operon that can occur independently of the trp repressor protein (trpR), suggesting it is a repressor-independent form of regulation. This process involves premature termination of transcription in response to tryptophan levels.

Question 3

How does tryptophan, the end product of the trp operon, function in the regulation of the operon?

Accepted Answer

Tryptophan acts as a corepressor in the trp operon. When tryptophan levels are high, it binds to the trp repressor protein, causing a conformational change that allows the repressor to bind to the operator region of the operon. This binding blocks RNA polymerase from transcribing the genes involved in tryptophan synthesis, thus regulating the production of tryptophan based on its availability.

Question 4

Which molecules are examples of effector molecules?

Accepted Answer

Effector molecules like allolactose and tryptophan (Trp) bind to regulatory proteins to modulate their activity, influencing gene expression.

Question 5

In E.coli the heat shock response, which alters the proteins produced by the cell at different temperatures, is mediated by

Accepted Answer

The heat shock response in E. coli is primarily mediated by the synthesis of alternative sigma factors at high temperatures, which then activate the transcription of heat-shock genes. These sigma factors redirect RNA polymerase to specific promoters of genes that help the cell cope with stress, such as those encoding for chaperones and proteases.

Question 6

How does a positive regulator affect transcription by RNA polymerase in prokaryotes?

Accepted Answer

Positive regulators (activators) in prokaryotes often function by directly interacting with RNA polymerase, thereby increasing the frequency of transcription initiation. This interaction can help stabilize the binding of RNA polymerase to the promoter or enhance its ability to begin transcription.

Question 7

In the trp operon, attenuation occurs through translation of two Trp codons in the leader sequence.What would happen if these two codons were mutated to stop codons?

Accepted Answer

The answer of In the trp operon, attenuation occurs through...

Question 8

The function of a helix-turn-helix motif in a transcription factor is to

Accepted Answer

The helix-turn-helix motif in a transcription factor is specifically designed to bind to a particular sequence of DNA within the major groove, allowing the transcription factor to regulate the expression of certain genes.

Question 9

Initiation of transcription by RNA polymerase involves the binding of which of the following subunits to the core enzyme?

Accepted Answer

The sigma factor is responsible for the initiation of transcription in bacteria by enabling the RNA polymerase to recognize and bind to specific promoter regions on the DNA.

Question 10

How do negative regulators such as the Lac repressor prevent RNA polymerase from initiating transcription?

Accepted Answer

The Lac repressor functions by physically blocking the DNA binding site of RNA polymerase, preventing it from initiating transcription at the lac operon.

Question 11

How is glucose involved in catabolite repression of the lac operon?

Accepted Answer

Glucose causes cAMP levels to decrease, which in turn leads to decreased CRP (cAMP receptor protein) binding to the lac operon. This results in repression of the lac operon even when lactose is present, as the CRP-cAMP complex is necessary for the activation of the operon.

Question 12

Transcription and translation can be coupled in bacteria but not eukaryotes because

Accepted Answer

In bacteria (prokaryotes), the lack of a nuclear membrane allows transcription and translation to occur simultaneously, as the mRNA being transcribed from DNA can immediately be translated by ribosomes. In contrast, eukaryotes have a nuclear membrane that separates transcription (in the nucleus) from translation (in the cytoplasm), preventing their coupling.

Question 13

Catabolic pathways that break down complex substances into more usable units are usually regulated in response to the

Accepted Answer

The answer of Catabolic pathways that break down complex substances...

Question 14

The transition from transcriptional initiation to elongation involves

Accepted Answer

The transition from transcriptional initiation to elongation in bacteria involves the release of sigma factor from the RNA polymerase complex. Sigma factor is crucial for initiation because it helps the RNA polymerase to recognize and bind to specific promoter regions on the DNA. Once transcription is initiated, the sigma factor is released, allowing RNA polymerase to proceed with elongation of the nascent RNA transcript.

Question 15

Proteins whose conformations change when they are bound to an effector molecule are called

Accepted Answer

Allosteric proteins change their conformation upon binding to an effector molecule, which can either enhance or inhibit their function. This is a key mechanism for regulating the activity of these proteins.

Question 16

A single DNA unit that enables the simultaneous regulation of more than one coding region in response to environmental changes is called a(n)

Accepted Answer

An operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and often share a common regulatory region, such as an operator. This allows the genes to be activated or repressed simultaneously in response to environmental changes.

Question 17

Which statement accurately describes global gene regulation in bacteria?

Accepted Answer

Alternative sigma factors allow bacteria to respond to different environmental conditions by recognizing different sets of promoters, thus enabling global changes in gene expression. This mechanism is crucial for adapting to stress, initiating sporulation, or shifting to a different metabolic state.

Question 18

What would be the phenotype of a null mutation in the gene encoding Lac repressor?

Accepted Answer

A null mutation in the gene encoding the Lac repressor would result in the inability to produce a functional repressor protein. This means that the lac operon would be expressed constitutively (all the time) because there is no repressor to inhibit its expression in the absence of lactose.

Question 19

Anabolic pathways involved in the synthesis of essential molecules are usually regulated in response to

Accepted Answer

Anabolic pathways are often regulated by feedback inhibition, where the end product of the pathway inhibits an enzyme involved in the pathway, thus controlling the synthesis of essential molecules.

Question 20

As a general principle of gene regulation through operons, regulatory genes encode

Accepted Answer

Regulatory genes encode trans-acting proteins, such as repressors or activators, which interact with cis-acting DNA elements like operators or enhancers to regulate the expression of other genes.

Quiz 16: Gene Regulation in Prokaryotes